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We now know of more than 1,000 planets orbiting other stars. In all likelihood, hundreds of billions more call the Milky Way home. Many of the known “exoplanets” are large, gaseous worlds like Jupiter or Neptune—hostile places for life.

But like those giants of our solar system, distant exoplanets may also have large moons. And if they do, moons—not planets—may be the most common home for life in the universe.